The present invention relates to a motor control system for properly controlling the reactive power of motor load equipment including a motor and a couple of power converters connected in series to regulate the power supplied to the motor.
in general, motor load equipment is operated while coupled with an AC power source. In this case, the motor load equipment is supplied with effective power and reactive power from the AC power source, so that the power factor of the power source is reduced. The reactive power consumed in the motor load equipment is lagging reactive power. To neutralize it, phase modifying equipment (for example, a condensor for improving the power factor) to consume leading reactive power is connected to the AC power source. Accordingly, the reactive power viewed from the power source side becomes small, so that the power factor of the power source is kept high.
However, the lagging reactive power consumed by the motor load equipment greatly varies depending on the operating condition. Particularly, in a light load running of the motor, the lagging reactive power consumed by the motor load equipment becomes small while the leading reactive power supplied from the phase modifying equipment becomes excessively large. In the case of the light load running, the phase modifying equipment excessively compensates the power factor, so that the power source voltage rises beyond a target value for regulation. In order to prevent such an abnormal rise of the power source voltage, a contactor by convention is provided between the phase modifying equipment and the power source, and in the light load running, the contactor is opened to disconnect the phase modifying equipment from the power source.
This method to connect or disconnect the phase modifying equipment to and from the power source by means of a contactor, however, fails to continuously control the reactive power.